Spinning seeds inspire single-bladed helicopters

A mini helicopter modelled on flying tree seeds could soon be flying overhead. Evan Ulrich and colleagues at the University of Maryland in College Park turned to the biological world for inspiration to build a scaled-down helicopter that could mimic the properties of full-size aircraft.

The complex design of full-size helicopters gets less efficient when shrunk, meaning that standard mini helicopters expend most of their power simply fighting to stay stable in the air. The researchers realised that a simpler aircraft designed to stay stable passively would use much less power and reduce manufacturing costs to boot.

It turns out that nature had beaten them to it. The seeds of trees such as the maple have a single-blade structure that allows them to fly far away and drift safely to the ground. These seeds, known as samaras, need no engine to spin through the air, thanks to a process called autorotation. By analysing the behaviour of the samara with high-speed cameras, Ulrich and his team were able to copy its design.

The samara copter is not the first single-winged helicopter – one was flown in 1952, and others have been attempted since – but it is the first to take advantage of the samara's autorotation. This allows Ulrich's vehicle to perform some neat tricks, such as falling safely to the ground if its motor fails or using vertical columns of air to stay aloft indefinitely. "We can turn off the motor and autorotate, which requires no power to sustain," says Ulrich.

Pitch piloting

The team found that a single-winged chopper is harder to control than a regular helicopter, because its constant spinning makes it impossible to work out which way the craft is orientated relative to its direction of flight using conventional sensors. What allowed Ulrich to take control was his discovery that both natural and robotic samaras travel in a fixed circle whose radius is determined by the pitch of the wing.

"We can change the size of the circle the vehicle is travelling in and pilot the aircraft to any desired destination," explains Ulrich.

Having built around 100 prototypes, Ulrich is ready to seek commercial applications for his artificial samara. Selling it as a toy is obvious appealing, but he has grander plans. "It can potentially compete with satellites," says Ulrich.

"A larger robotic samara would be deployed into the jet stream and be able to use the fast-moving air to stay aloft." A samara-mounted camera could also be used to create 3D maps of terrain below or in a war could relay images to soldiers on the ground.

"The samara concept is an interesting one, which has merit in terms of its size and mass," says Stephen Prior, a researcher in autonomous robotic systems at Middlesex University in London who warns that the vehicle's spin might make it difficult to take pictures in flight. "However, like most nano air vehicles it suffers from lack of endurance and the difficulty of getting a stable image off the platform, since it is continuously spinning."

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